Humans are an odd species. We live in colossal societies, prune our body hair according to current fashions, and communicate in mutually unintelligible languages. Perhaps most curious though is the ubiquity of religion in human life. Across cultures, people revere supernatural beings ranging from humble tree spirits to powerful monotheistic deities, and engage in supernatural practices ranging from discreet garden magic to ostentatious ritualised human sacrifices. How could such a bewildering array of supernatural beliefs and practices arise? How broadly do religious systems affect the way people think about the world? Can religion serve functions or does it simply impose costs?